This invention is concerned with reference electrodes and in particular with a novel salt bridge and liquid junction structure for such electrodes which comprises anionic and cationic polymer.
The reference electrode is a necessary part of instrumentation systems used in the measurement of electrochemical potentials in analytical laboratories and industrial process control systems. The stability of the reference electrode potential is often the major factor which determines the precision of the measurement. In order to maintain this stability the internal reference cell must be isolated from direct contact with the various test solutions and kept in a constant ionic environment. This is accomplished by use of a salt bridge which is usually a strong solution of potassium chloride and in which the reference cell is immersed. The salt bridge solution must have a constant concentration and must also function to make electrochemical contact with the test sample and thus complete the electrical circuit between the reference and sensing electrodes. The salt bridge makes contact with the test solution through some form of porous substance or capillary structure through which the salt bridge solution flows or diffuses slowly.
Many different types of structures have been used to make this liquid junction. A porous plug of material, such as ceramic, wood, porous Teflon, quartz fiber, or a micropore structure is used as the liquid junction. The sample electrolytes make contact either by slow flow of salt bridge solution through the porous structure into the sample, a flowing junction electrode, or by mutual diffusion into the porous structure from each side, a sealed static junction electrode.
The flowing junction electrode is the more stable and reproduceable of these two junction types. When properly designed it performs reliably in most test samples and is preferred for high precision laboratory measurements. It must be maintained, however, by refilling the salt bridge body with electrolyte to restore the solution lost by outflow. This is not a serious inconvenience in laboratory applications. However, in continuous monitoring for industrial pH control it can become a substantial cost factor, both because of the elaborate hardware needed and the necessity for well-trained personnel.
The sealed, static junction system is designed to be maintenance free for the useful life of the salt bridge. These electrodes have the advantage of requiring simple hardware and minimal maintenance. The salt bridge in such electrodes cannot be maintained by the replenishment of the salt bridge electrolyte and will eventually fail due to contamination and/or dilution of the salt bridge. Although electrodes of this design are not as precise and reproduceable as those with flowing junctions, they have an acceptably long life in many routine laboratory measurements where critical precision is not needed. However, situations where temperature and pressure fluctuations occur will greatly shorten electrode life. A few industrial pH control applications exist where these electrodes perform well enough for an acceptable period of time. These are applications where sample composition is not chemically destructive and where temperature and pressure remain relatively constant.
An electrochemical reference electrode must ideally have a constant potential in a variety of test solutions in order for the electrode to qualify as a reference. However, in practice, most reference electrodes do not behave in this manner and often deviate from one given test solution to another. This deviation can often be compensated for by adjustment of the metering controls so as to bring the value back to the apparent constant potential. However, this is clearly no substitution for a better reference electrode and sometimes the deviation is so great that it cannot be corrected for by the range available on the control unit.
Also, the response time to obtain a stable reading in a test sample should be rapid, for convenience. Long response times tend to lead to premature, and therefore inaccurate, readings. Conventional sealed electrodes tend to be relatively slow. In continuous monitoring situations the reading should be as consistent as possible to obtain an accurate measurement. Often conventional reference electrodes drift with time and therefore give uncertain and inaccurate measurements. When calibrating the electrode error is easily introduced by using the first calibration measurement to establish calibration in other samples. In fact, because of the hysterysis effect, prominent in sealed electrodes, the first calibration value could be varying and therefore this will render the other values inaccurate.
Some test samples provide particular difficulties for the reference electrode. In particular, it is extremely difficult to obtain an accurate and consistent pH reading in high purity, deionized water. In boiler feed waters using such water, it is essential that the pH of the water be monitored. Otherwise the boilers become corroded and this requires expensive maintenance and eventually even replacement of the boilers. Moreover, the reference electrode used in such an environment must be reasonably easy to operate and maintain by the personnel involved. For the reasons mentioned above, flowing junction electrodes are awkward to use and maintain. If not maintained properly, particularly in high purity water, the sample ions tend to rapidly penetrate the electrode and make its reading unstable and eventually useless. The intrusion of high purity water, for example, creates a high impedence in the liquid junction. Sealed electrodes irrespective of maintenance suffer from this intrusion of the sample. Measurement of pH in such water has proved so difficult that procedures have been developed which involve adding to the water a trace amount of electrolyte in order to obtain even an approximately stable measurement. This is obviously self-defeating in that the addition of ions to deionized water changes the pH of the water and provide a reading which is not completely representative of the water itself.
The use of charged polymers in electrodes is known. Carson in Anal. Chem. 27, 472-3 (1955) discloses the preparation of electrodes with an anionic ion-exchange membrane or a cationic ion-exchange membrane. Patnode in U.S. Reissue No. 24222 also discloses certain cationic ion-exchange membranes. Niedrach discloses a carbon dioxide sensor in U.S. Pat. No. 3,730,868 and an oxygen sensor in U.S. Pat. No. 3,703,457 with anionic ion-exchange resin between the electrodes and an oxygen sensor in U.S. Pat. No. 3,800,410 with an anionic ion-exchange resin or a cationic ion-exchange resin between the electrodes. Niedrach also discloses in French patent no. 2,158,905 an ion specific membrane electrode in which the electrolyte comprises an immobilized anionic ion-exchange resin. Freiser in U.S. Pat. No. 4,115,209 discloses an electrode in which a coating of anionic ion-exchange resin is prepared on a conductive surface. Chang in U.S. Pat. No. 4,282,079 discloses a planar, multilayer electrode with a hydrophobic ion-exchange resin in the electrolyte layer.
Despite these known structures and the wide variety of other known electrodes, the above-described problems still exist. There is therefore a need for an electrochemical reference electrode which provides accurate reference measurement in a variety of test samples, provides long-term stable readings in difficult samples such as high-purity, deionized water and which requires low maintenance. Such an electrode has now been discovered.